‘Oumuamua as a light sail: Evidence against artificial origin

‘Oumuamua, the first detected interstellar visitor to the Solar System, exhibits non-gravitational acceleration in its trajectory. In ruling out other means of propulsion, such as the evaporation of material via a cometary tail, it has been argued that radiation pressure is responsible for this acceleration. From this (a = 5 × 10−6 m s−2), the mass of the object must be approximately 4 × 104 kg, and given its dimensions, ‘Oumuamua must have a thickness of ≲1 mm if of a similar rock and iron composition as the Earth. This raises the much publicised possibility that ‘Oumuamua is artificial in origin and intentionally sent across interstellar space by an alien civilisation. This conclusion, however, relies upon the common misapprehension that light (solar) sails can accelerate to a considerable fraction of the speed of light, permitting rapid interstellar travel. We show that such speeds are unattainable for conceptual man-made sails and that, based upon its observed parameters, ‘Oumuamua would require half a billion years just to travel to our Solar System from its closest likely system of origin. These cosmological timescales make it very unlikely that this is a probe sent by an alien civilisation.